Please don't be scared, I have been on Humira for about 3 years now and haven't seen any side effects. Before that I was on Infliximab, but I started to develop antibodies to it and had to change.
These drugs really are the top draw when it comes to treating Crohn's and getting you to remission. They are expensive and wouldn't be prescribed without due care and attention given. Yes there are risks, but there are risks with any medicine but as Scipio mentioned the risks of untreated Crohn's is a lot higher.
I know only too well what happens when Crohn's turns nasty. I had a perforation, which in turn burst my appendix and I ended up with peritonitis. I had temperatures so high that I actually felt freezing and couldn't stop shivering. I was in hospital for 6 months. I left weighing 6 stone (that's 84 pounds for our American cousins!).
I don't want to frighten anyone, but these things can happen, rarely but they do, Your Crohn's is never going to go away, it is something that you will have for the rest of your life.
I wish I was given the chance of going on biological when I was first diagnosed, it would of probably saved me and my family a lot of heartache and pain